Assuming we are dealing with identical amps, would it not be better to bridge each into a mono amp? In such a case the power would be more than doubled. Someone more technically astute than me could explain why and how much power would then, at least theoretically, be available. I am not certain bridging is always feasible though. Sounds like what you are attempting is more like bi-amping, which can be done with a number of speakers; again this requires two identical amps and speakers that can be bi-amped (the crossover actually being in two sections that can be disconnected). Again someone with a better technical background than mine would be called for.
